1. Technical Field of the Invention
The present invention relates to gas sensing elements and, more particularly, to a gas sensing element having a sensor cell for detecting a specified gas concentration in measuring gases admitted to a measuring gas chamber and an oxygen pump cell for adjusting an oxygen concentration in the measuring gas chamber.
2. Description of the Related Art
Attempts have been made to install gas sensors on exhaust systems of, for instance, automotive vehicles for measuring a specified gas concentration in measuring gases such as a NOx concentration in exhaust gases.
The gas sensors usually include gas sensing elements, one of which is disclosed, for instance, in FIG. 26 (which is extracted from Patent Publication: Japanese Patent Application No. H8-271476). The gas sensing element 9 includes a sensor cell 93 for detecting a specified gas concentration in measuring gases, and an oxygen pump cell 92 for adjusting an oxygen concentration in a measuring gas chamber 91.
Due to an adverse affect caused by oxygen contained in measuring gases, it is likely that a difficulty arises in accurately measuring a concentration of specified gas to be measured in nature. In view of such a difficulty, the gas sensing element 9 allows the oxygen pump cell 92 to adjust the oxygen concentration in the measuring gas chamber 91 to suppress such an adverse affect.
As shown in FIG. 26, more particularly, the gas sensing element 9 includes a plurality of solid electrolyte bodies 95 between which a first inner empty space 911 and a second inner empty space 912 are defined. The first inner empty space 911 admits measuring gases through a first diffusion rate-controlling passage 961 formed in one of the solid electrolyte bodies 95. The second inner empty space 912 communicates with the first inner empty space 911 via a second diffusion rate-controlling passage 962.
The sensor cell 93 includes a measuring electrode 931 exposed to the second inner empty space 912. Likewise, the oxygen pump cell 92 includes an inner pump electrode 921 exposed to the first inner empty space 911. In addition, the sensor cell 93 also includes a reference electrode 932, acting in pair with the measuring electrode 931, which is exposed to a reference gas chamber 913. Moreover, an outer pump electrode 922, acting in pair with the inner pump electrode 921, is located on the gas sensing element 9 at an outer area thereof.
Further, the measuring electrode 931 of the sensor cell 93 is located in the second inner empty space 912 and the inner pump electrode 921 of the oxygen pump cell 92 is located in the first inner empty space 911. Furthermore, the reference electrode 932, acting in pair with the measuring electrode 931, is located in the reference gas chamber 913. Moreover, the outer pump electrode 922, acting in pair with the inner pump electrode 921, is located on the gas sensing element 9 at the outer area thereof.
Furthermore, the first inner empty space 911 encompasses an inner monitor electrode 941 of an oxygen monitor cell 94 for detecting the oxygen concentration in the first inner empty space 911. The oxygen monitor cell 94 is comprised of the inner monitor electrode 941 and the reference electrode 932, between which an electromotive force occurs to allow the oxygen concentration to be detected in the first inner empty space 911.
With the gas sensing element 9 of such a structure, measuring gases are admitted to the first inner empty space 911 via the first diffusion rate-controlling passage 961. When this takes place, the oxygen pump cell 92 pumps out oxygen in measuring gases to the outside, causing a reduction in oxygen concentration in measuring gases. Measuring gases, lowered in oxygen concentration, are admitted to the second inner empty space 912 via the second diffusion rate-controlling passage 962.
The sensor cell 93, having the measuring electrode 931 located in the second inner empty space 912, measures a specified gas concentration in measuring gases. When this takes place, the oxygen concentration in measuring gases is adequately lowered with a resultant decrease in adverse affect caused by the oxygen concentration, thereby minimizing an error in measuring the specified gas concentration.
Meanwhile, the oxygen monitor cell 94 monitors the oxygen concentration in measuring gases admitted to the first inner empty space 911. The oxygen pump cell 92 provides an output varying depending on a resulting measured result of the oxygen concentration for thereby varying a capacity of pumping oxygen. This allows the oxygen concentration in measuring gases to be maintained at a fixed level in the first inner empty space 911.
However, the gas sensing element 9 of such a related art encounters various issues as described below.
That is, the measuring gas chamber 91 of the gas sensing element 9 is separated into the first inner empty space 911 and the second inner empty space 912 and juxtaposed along a longitudinal direction Y of the gas sensing element 9. The first diffusion rate-controlling passage 961, acting to admit measuring gases, is formed in the solid electrolyte body 95 at an end portion of the first inner empty space 911 in opposition to the second inner empty space 912 so as to extend in a stack direction Z. Thus, the measuring gases, admitted through the first diffusion rate-controlling passage 961, need to flow in a long distance to reach the measuring electrode 931 of the sensor cell 93 in the second inner empty space 912. As a consequence, the gas sensing element 9 has a risk of a drop in response.